Commitments and Contingencies - Collaborative Agreements, Royalties and Milestone Payments (Details) $ in Millions	Dec. 31, 2019 USD ($) product
Commitments And Contingencies
Estimate of milestone payments \| $	$ 7.6
Assumed number of products per contract \| product	1

Potential estimated obligations that due and payable upon achievement of certain criteria categorized as developmental, regulatory, or commercial milestones.

No definition available.

Name:	xoma_ContractualObligationEstimatedMilestonePayments
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant